Dalton stated in his atomic theory that atoms could neither
be created or destroyed. From a chemical perspective all mass
is contained by atoms. Therefore, if atoms cannot be created
or destroyed then neither can mass. This is known as the principle
of "Conservation of mass". With this principle, important
information about a chemical reaction can be obtained. For example,
consider the combustion of magnesium metal:

In a laboratory we carefully measure 10g of the metal, set
it on fire in the presence of air (combust it) and then carefully
weight the ash. We find that the ash weighs 16.6g. Using the
principle of conservation of mass we conclude that 16.6g - 10g
= 6.6g of oxygen reacted with the magnesium. A similar calculation
can be performed on all chemical reactions.